Overview of a Special Publication on Transmission System Application Requirements for FACTS Controllers

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1 1 Overview of a Special Publication on Transmission System Application Requirements for FACTS Controllers D. G. Ramey, Fellow, IEEE, M. Henderson, Sr. Member, IEEE Abstract--This paper describes an IEEE Special Publication. It was prepared for a Panel session on power system planning and FACTS applications. The special publication discusses system planning issues and requirements for applications of FACTS Controllers into electric transmission networks. It lists applications and discusses differences between traditional equipment and FACTS Controllers. Characteristics of models for FACTS Controllers for system planning and analysis software are described. Index Terms--Power system planning, Power system control, Power transmission control, Static VAR compensators, Voltage control F I. INTRODUCTION ACTS, an acronym which stands for Flexible AC Transmission System, is an evolving technology-based solution envisioned to help the utility industry to deal with changes in the power delivery business. The term FACTS refers to alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability. Technology concepts were conceived in the 1980 s and projects sponsored by the Electric Power Research Institute (EPRI) demonstrated many of these concepts with laboratory scale circuits. In the early 1990 s development of higher power electronic switching devices had progressed to the point that utility size installations were feasible. It is hoped that growth in demand for these products will spur continued development in the power electronic devices allowing larger sizes and more economical installations. The working Group of the HVDC and FACTS Subcommittee was formed to help the electric utility industry define applications where FACTS controllers are most appropriate. It was soon apparent that defining applications for FACTS Controllers requires an understanding of the planning process for transmission systems. These controllers must function and be economical in a deregulated market environment as well as meet the traditional requirements for Don Ramey is retired and a part time consultant ( donramey@ieee.org). Mike Henderson is with ISO New England, USA ( mhenderson@isone.com). system security, reliability and sufficient capacity to meet the needs of customers. Therefore the focus of the special publication became a discussion of the changing utility environment, the choices of solutions for power transfer and voltage control issues and the system requirements that FACTS Controllers must meet. In addition to describing the planning process, functions performed by each of the FACTS Controllers are briefly described and their basic circuits shown. This publication assumes the availability of models and the associated planning software and focuses on how the transmission system planning process can define and justify FACTS applications.. The main sections of the special publication are:. Transmission Planning Environment. Transmission Planning and System Control. Overview of FACTS Controllers. FACTS Controller Models. Operational Requirements for FACTS Controllers Over the several years that this special publication has been discussed and revised there have been a number of contributors. Some have attended committee meetings and offered constructive comments while others have supplied technical material for the publication and helped edit and refine the document. The contributors identified in the acknowledgements of the publication are Ram Adapa, Jacques Brochu, Kara Clark, Abdel-Ati Edris, Edi von Engeln, Carlos Gama, Brian Gemmel, John Hauer, Mike Henderson, Narian Hingorani, Brian Johnson, Ben Mehreban, John Mountford, John Paserba, Don Ramey, Hector Sarmiento, Colin Schauder, Rajiv Varma and Dennis Woodford. The chairman of this working group throughout the 4-year period during which this guide was developed was Mike Henderson. II. FACTS CONTROLLER APPLICATIONS The simplest way to identify the potential roles to be played by FACTS Controllers is to examine their functions as they relate to conventional equipment. The definition of FACTS systems incorporates both power electronic-based and other static controllers to enhance controllability and increase power transfer capability. One of the system planners tasks is to determine which combinations of Controllers provide both the capacity to supply the reactive power, dynamic reserve and continuous regulation needed for the application. Table 1 lists /07/$ IEEE.

2 2 the main functions that can be performed by FACTS Controllers and show both FACTS and other conventional equipment that performs these functions. A characteristic of FACTS controllers is the ability to have control algorithms structured to achieve multiple objectives. Since FACTS Controllers have control systems with embedded digital processors, it is possible to switch between control algorithms and to include different types of nonlinear limiting functions. Both shunt connected and series connected controllers can be programmed to assist in one or more of these functions. They will have varying effectiveness depending on the power rating of the individual controller; it s location in the network, and the desired function. In addition to the primary control function, these controllers can also provide damping to system oscillations. The effectiveness of this damping control is highly dependent of the location of the controller in the network. The value of FACTS applications lies in the ability of the transmission system to reliably transmit more power or to transmit power under more severe contingency conditions with the control equipment in operation. If the value of the added power transfer over time is compared to the purchase and operational costs of the control equipment, relatively complex and expensive applications may be justified. Other economic considerations include the market structure, transmission tariff, and identification of winners and losers. Realization of the value added by a proposed transmission project often requires a coordinated implementation of conventional transmission equipment, possibly including transmission line segments, FACTS Controllers, coordinated control algorithms and special operating procedures. Table 1 System Control Functions Function Non FACTS Control Methods FACTS Controllers Voltage Control Electric generators Synchronous Condensers Conventional Transformer tap-changer Conventional Shunt Capacitor/Reactor Conventional Series Capacitor/Reactor Static Var Compensator (SVC) Static Synchronous Compensator (STATCOM) Unified Power Flow Controller (UPFC) Superconducting Energy Storage (SMES) Battery Energy Storage System (BESS) Convertible Static Compensator (CSC) a full scale collaborative projects sponsored by EPRI in conjunction with a host utility and an equipment supplier. The Controllers described in the special publication are: Static Var Compensator (SVC) Static Synchronous Compensator (STATCOM) Superconducting Magnetic Energy Storage (SMES) Battery Energy Storage System (BESS) Thyristor Controlled Series Capacitor (TCSC) Static Synchronous Series Compensator (SSSC) Unified Power Flow Controller (UPFC) Interphase Power Controller (IPC) The Static Var Compensator used for transmission system applications is a dynamic source of leading or lagging reactive power. It is comprised of a combination of reactive branches connected in shunt to the transmission network through a step up transformer. The SVC is configured with the number of branches required to meet a utility specification as indicated in Figure 1. This specification includes required inductive compensation and required capacitive compensation. The sum of inductive and capacitive compensation is the dynamic range of the SVC. One or more thyristor-controlled reactors may continuously vary reactive absorption to regulate voltage at the high voltage bus. This variation is accomplished by phase control of the thyristors, which results in the reactor current waveform containing harmonic components that vary with control phase angle. A filter branch containing a power capacitor and one or more tuning reactors or capacitors is included to absorb enough of the harmonic currents to meet harmonic specifications and provide capacitive compensation. The thyristor switched capacitor is switched on or off with precise timing to avoid transient inrush currents. Trans. Active and Reactive Power Flow Control Transient Stability Dynamic Stability Short Circuit Current Limiting Generator schedules Transmission line switching Phase Angle Regulator (PAR) Series Capacitor (switched or fixed) High Voltage Direct Current Transmission (HVdc) Braking Resistor Excitation Enhancement Special Protection Systems Independent Pole Tripping Fast Relay Schemes Fast Valving Line Sectioning HVdc Power System Stabilizer HVdc Switched series reactors Open circuit breaker arrangements Interphase Power Controller (IPC) Thyristor controlled Series Capacitor (TCSC) Thyristor Controlled Series Reactor (TCSR) Thyristor Controlled Phase Shifting Transformer (TCPST) UPFC Static Synchronous Series Compensator (SSSC) Interline Power Flow Controller (IPFC) Thyristor Controlled Braking Resistor (TCBR) SVC, STATCOM, TCSC, TCPST, UPFC BESS, SMES, SSSC, CSC, IPFC TCSC, SVC, STATCOM, UPFC, SSSC, TCPST, BESS, SMES, SSSC,CSC, IPFC Thyristor switched series reactor, TCSC, IPC, SSSC, UPFC; These are secondary functions of these controllers and their effectiveness may be limited. TCR TSC Filter Figure 1 Circuit diagram of a SVC containing a thyristor controlled reactor, a thyristor switched capacitor and a double tuned filter III. OVERVIEW OF FACTS CONTROLLERS This section of the special publication shows the circuit diagram and describes the operation of the FACTS Controllers that have either been commercially applied or demonstrated in The STATCOM shown in Figure 2 performs the same voltage regulation and dynamic control functions as the SVC. However, its hardware configuration and principle of

3 3 operation are different. It uses voltage source converter technology that utilizes power electronic devices (presently gate turn-off thyristors (GTO), GCTs or insulated gate bi-polar transistors (IGBT)) that have the capability to interrupt current flow in response to a gating command. Analogous to an ideal electro magnetic generator, the STATCOM can produce a set of three alternating, almost sinusoidal voltages at the desired fundamental frequency with controllable magnitude. The angle of the voltage injected by the STATCOM is constrained to be very nearly in-phase with the transmission network at the point of connection of the coupling transformer. When the voltage is higher in magnitude than the system voltage, reactive current with a phase angle 90 degrees ahead of the voltage phase angle flows through the coupling transformer. This is analogous to the operation of a shunt capacitor. When the generated voltage is lower than system voltage, the current phase angle is 90 degrees behind the voltage phase angle that is analogous to the operation of a shunt reactor. The slight deviation in voltage phase angle absorbs power needed for the losses in the circuit. For high power applications a number of six or twelve pulse converters are operated in parallel to meet both the current rating requirement and the harmonic requirement of the network. Two different switching patterns, phase displaced converters with electronic devices switched once per cycle and pulse width modulation, have been used to form the sinusoidal waveform. conventionally switched series capacitors. Figure 3: One Line Diagram of the TCSC A static synchronous series compensator (SSSC) is connected in series with a transmission line and is comprised of a voltage source converter operated without an external electric energy source. (See Figure 4) This configuration serves as a series compensator whose output voltage is in quadrature with, and controlled, independently of the transmission line current. Figure 4: Circuit diagram for a Static Synchronous Series Compensator Figure 2: STATCOM circuit diagram The thyristor controlled series capacitor (TCSC) is placed in series with a transmission line and is comprised of three parallel branches: a capacitor, a thyristor pair in series with a reactor (TCR), and a metal oxide varistor (MOV) that is required to protect against overvoltage conditions. (See Figure 3. The TCSC can function as a series capacitor if the thyristors are blocked or as a variable impedance when the duty cycle of the thyristors is varied. Applications of TCSCs currently in service provide impedance variations to damp inter-area system oscillations. The most economical installations often contain one segment of thyristor-controlled capacitors in series with one or more segments of The purpose of the SSSC is to increase or decrease the overall reactive voltage drop across the line and thereby control the transmitted real electric power. The SSSC may include transiently rated energy storage or energy absorbing equipment to enhance the dynamic behaviour of the power system by additional temporary real power compensation, to increase or decrease momentarily, the overall real (resistive) voltage drop across the line. This action controls the reactive power flow on the line. The Unified Power Flow Controller (UPFC) provides voltage, and power flow control by using two high power voltage source converters (VSC) coupled via a dc capacitor link. Figure 5 shows the two interconnected converters. VSC 1 is connected like a STATCOM and VSC 2 is connected as a SSSC in series with the line. With the dc bus link closed, the UPFC can simultaneously control both real and reactive power flow in the transmission line by injecting voltage in any phase

4 4 angle with respect to the bus voltage with the series converter. The shunt-connected converter supplies real power required by the series connected converter. With its remaining capacity the shunt converter can regulate bus voltage The UPFC circuit can be reconfigured by use of external switches and possibly additional transformers to form STATCOM, SSSC, or coupled SSSC circuits. Similar to other inverter based FACTS Controllers, FACTS controllers that contain energy storage are coupled to the AC network through an AC-DC inverter. In addition they have a DC-DC power circuit to interface the energy storage (to date either a superconducting magnet or a battery) to the DC bus of the inverter. This equipment has been applied at the distribution voltage level. Energy limitations for storage systems have limited applications to short-term backup for critical loads and to dynamic damping of system oscillations during transient conditions. Line VSC 1 dc bus Control VSC 2 Figure 5: Circuit Diagram of a Unified Power Flow Controller IV. FACTS CONTROLLER MODELS For FACTS Controllers to be included in transmission system plans, there must be appropriate models for all the analyses that are normally performed. To date only the SVC typically has an embedded model in the most widely used power flow (load flow) software. Some of the other controllers are represented by user defined models and others by models for electric machines or static inductors and capacitors. This lack of explicit models extends to the software used in many transmission control centers and it is an impediment to defining new applications and to operating FACTS equipment. Models for dynamic simulation studies have, to date, been made by the equipment suppliers. These models often represent the FACTS Controllers in extreme detail requiring simulation time steps that are too small for the models to be easily incorporated into software that simulates large electric networks. There is a need for more general models that are compatible with the simulation software that is more widely used. These models would:. Represent the power circuit equations algebraically in the appropriate software routines.. Represent inverters as voltage sources.. Allow voltage changes that affect the network to occur at simulation time steps rather than continuously.. Represent control functions using differential equation and logic algorithms For specialized studies including harmonic analyses and analysis of switching phenomenon there must be models that are compatible with EMTP or EMTDC analysis software. These models must be made with the cooperation of the equipment manufacturer. They can represent inverters either as AC voltage sources or by detailed switching circuits. If the switching circuit is employed, the model must also represent the switching control logic including phase locked loop synchronizing circuits. The logic that protects inverter valves from overcurrents must also be included in these models. This level of detail is normally required only for design studies or for detailed analysis of operating issues with the equipment. V. OPERATIONAL REQUIREMENTS FOR FACTS CONTROLLERS When a utility, transmission coordinating council, or regional transmission organization (RTO) considers the addition of FACTS Controllers, the consideration usually involves a number of system requirements to assure the reliability and security of the installation. Since most of the FACTS Controllers contain a computer based control system, they can be programmed to both perform their primary function and also manage the operation of conventional transmission equipment. Formalized procedures must be developed to define system conditions where coordinated operation of the FACTS Controller and other transmission equipment is needed. These formalized procedures are often published and increasingly are available to other system planners as well to the general public. These controllers are expected to function during both normal and transient conditions in the electric system. To meet this expectation requires design and certification procedures based upon:. Directly measured system dynamics.. Assured resources for the prompt detection, analysis and correction of anomalous controller effects.. Performance monitors that communicate information to system analysts. Tests for commissioning and periodic certification. Information exchange among grid operators. Although these formalized guidelines are necessary to assure the security of the network, they place significant demands for information about component reliability and system reliability for FACTS Controllers. They also place demands for careful study of interactions in the transmission system and definition of system contingencies that most stress the application. Much of the required information is not well known during the development phase of a FACTS Controller and engineering judgment and cooperation between equipment designers and system engineers is essential in early

5 5 applications. For FACTS Controllers to become widely used they must:. Meet the availability and maintenance requirements that are expected for other power electronic equipment used in the system.. Contain operator interface software and displays that clearly show the operating state of the Controller and allow the operator to readily change reference settings, control modes or limits.. Meet automatic startup, shutdown and mode change requirements. VI. CONCLUSIONS Although in the deregulated electric system environment, transmission system planning is more difficult, the industry has always sought the application of equipment that will maximize the use of available transmission lines. FACTS Controllers are just additional options available to the planning engineer. Specifically, they are a new generation of power electronic based equipment with the same function as conventional equipment but with enhanced controllability and speed of response. Traditional planning methods still apply. Equipment selection will depend on function, availability, cost, applicability, reliability, and robustness in the face of future uncertainties. Transmission networks operating at current flow levels near the thermal limits of transmission lines require large amounts of reactive power. They also require that this reactive power is properly distributed throughout the network and that a portion be dynamic to prevent voltage collapse during system contingencies. The allowed transmission limits are defined both by rules intended to meet reliability requirements and the physical limits of the system. The value of FACTS Controllers increases as the operating limits of the system approach the physical limits. The Special Publication for System Planners IEEE WG , Transmission System Application Planning Requirements for FACTS Controllers provides guidance on how to incorporate FACTS Controllers into the traditional planning process. It includes detailed discussions of the various types of FACTS Controllers, their functions and applicability, as well as commentary on appropriate models for the necessary planning analyses. With these tools, the transmission planner now has additional options available to improve overall transmission system usage while maintaining system reliability. VIII. BIOGRAPHY Donald G. Ramey (M 1962, F 1995) graduated from Colorado State University with a BSEE in He received a MSEE from the University of Pittsburgh in 1965 and was a Fellow at the Center for Advanced Engineering Study at Massachusetts Institute of Technology in He worked for Westinghouse Electric Corporation from 1964 until Siemens purchased the Power Generation Division in 1998 and retired from the Siemens Power Transmission and Distribution Division in He held engineering and engineering management positions that developed power system simulation software and provided engineering consultation on systems applications of electrical equipment and resolution of technical issues. Later in his career Don was manager of electrical development for large electric generators and exciters. He also managed system engineering and system applications for FACTS products. Michael I. Henderson received two ME degrees, Electrical Power Engineering in 1977 and Electrical Engineering in 1976, both from Rensselaer Polytechnic Institute, USA. In 1975, he earned a BS in Electrical Engineering from Polytechnic Institute of New York where he served as an Adjunct Lecturer from 1993 through Since July 1999, Mike has been the Director, System Planning Department at the ISO-New England. Previously, he had more than 22 years experience at the New York Power Authority, Long Island Lighting Company, and American Electric Power. Mike has presented technical seminars as well as over three-dozen panel and technical papers at IEEE and other forums. Mike is a registered Professional Engineer and he is a native son of Brooklyn, New York. VII. REFERENCES [1] A Special Publication for System Planners IEEE WG , Transmission System Application Planning Requirements for FACTS Controllers, 2006

Overview of Flexible AC Transmission Systems

Overview of Flexible AC Transmission Systems What is FACTS? Flexible AC Transmission System (FACTS): Alternating current transmission systems incorporating power electronic-based and other static controllers